Burner of use with fluid fuels

ABSTRACT

A burner for fluid fuels comprises a fuel chamber, a plurality of combustion air tubes which pass through the fuel chamber and a packing positioned between the combustion air tubes and adjacent to the combustion zone. The packing divides the fuel chamber into an unpacked inlet zone into which the fuel first passes and a packed outlet zone through which the fuel passes on its way to the combustion zone. For gaseous fuels the packing is a high outlet resistance so that even fuel distribution is achieved in the unpacked zone. For liquid fuels the packing acts as a wick and assists in heat transfer to vaporise the fuel into the combustion zone.

1 Dec. 2, 1975 United States Patent [191 Desty et al.

1 BURNER OF USE WITH FLUID FUELS [75] Inventors: Denis Henry Desty,

waltomonqhames; David Montagu Primary Examinew-Carroll B. Dority, Jr. Whitehead, Cambefley, both f Attorney, Agent, or FirmMorgan, Finnegan, Pine,

Foley & Lee

England [57] ABSTRACT A burner for fluid fuels comprises a fuel chamber, a

[73] Assignee: The British Petroleum Company Limited, London, England Oct. 8, 1968 [22] Filed:

plurality of combustion air tubes which pass through PP 765,909 the fuel chamber and a packing positioned between the combustion air tubes and adjacent to the combustion zone. The packing divides the fuel chamber into [30] Foreign Application Priority Data Oct. 10, 1967 United Kingdom............

even fuel distribution is achieved in the unpacked zone. For liquid fuels the packing acts as a wick and assists in heat transfer to vaporise the fuel into the combustion zone.

17 Claims, 9 Drawing Figures 372,518 Clayton............................... 431/326 US. Patent Dec. 2, 1975 Sheet 1 of3 3,923,447

. 30 INVENTORS,

DENIS HENRY DESTY BY DAVID MONTAGU WHITEHEAD MORGAN, FINNEGAN, DURHAiM 8r PINE ATTORNEYS US. Patent Dec. 2, 1975 She et 2 3,923,447

PIC-3.6.

INVENTORS,

DENIS HENRY DESTY DAVID MONTAGU WHITEHEAD MORGAN, FINNEGAN, DURHAM 8: PINE ATTORNEYS U.S. Patent Dec. 2, 1975 Sheet 3 0f3 3,923,447-

INVENT'ORS,

DENIS HENRY DESTY DAVID MONTAGU WHITEHEAD MORGAN, FINNEGAN, DURHAM 8 PINE ATTORNEYS BURNER OF USE WITH FLUID FUELS This invention relates to a burner for use with fluid fuels, i.e. to a burner suitable for use with liquid and gaseous fuels.

It would be convenient for the manufacturers of appliances which incorporate burners to have available a type of burner which enables a wide range of fuels to be burnt in burners of similar construction, size and with similar heat outputs. In particular it would be convenient to have available a gas burner which, without modification, can burn both high and low speed gases, e.g. methane and hydrogen.

According to the invention a burner for fluid fuels comprises a plurality of combustion air tubes adapted to convey combustion air to a combustion zone, said tubes passing through a fuel chamber which is divided into a sequence of:

a. at least one unpacked fuel space which provides a low resistance to the flow of fuel across the burner and b. at least one packed fuel space in which the packing is adapted to control the flow of fuel towards the combustion zone,

the first of said fuel spaces (hereinafter called the fuel inlet space) being unpacked and adapted to be connected to a fuel supply and the last fuel space of said sequence (hereinafter called the fuel outlet space) being packed and communicating with the combustion zone whereby, during the use of the burner, air flows through the combustion air tubes and into the combustion zone where it reacts with the fuel which flows through the sequence of unpacked and packed spaces and finally into the combustion zone. Preferably the burner comprises two fuel spaces, namely the fuel inlet space and fuel outlet space.

The packing is conveniently supported on a plate which extends across the fuel chamber and which permits passage of fuel. Examples of such plates include gauzes, perforated plates and plates which provide annular fuel passages around the combustion air tubes. In certain cases the packing may have sufficient mechanical cohesion to render the use of a plate unnecessary.

Cylindrical tubes are particularly suitable as the combustion air tubes. Most suitably the combustion air tubes are arranged with their axes parallel to one another.

The flow of fuel from a burner as described above tends to be aligned with the flow of air from the combustion air tubes. This gives satisfactory combustion but where very high fuel flow rates are required better combustion may be achieved if the fuel flow is deflected into the combustion air flowing out of the combustion air tubes. Where this is required the burner may comprise a baffle positioned over the fuel outlet space.

The following two constructions are particularly suitable for use at the inlet end of the combustion air tubes:

Construction A The combustion air tubes are secured in fluid tight manner into holes in an air inlet zone plate which forms one wall of the fuel inlet space.

Construction B The combustion air tubes have a conformable polygonal cross-section, e.g. equilateral triangles, squares or regular hexagons and the walls of the polygons are secured to one another in fluid tight manner.

In the case of gaseous fuel burners the packed fuel spaces provide a relatively high resistance to the flow of fuel (this implies that no low resistance channels are left, e.g. around the combustion air tubes) and the combination of low and high resistance encourages an even fuel distribution. The packing is preferably a porous material such as a particulate material, e.g. a powder whose particle size and particle density is such as to produce the required high resistance to fuel flow. The particles may be bound together, e.g. by pressure sintering, heat sintering, the use of a binder or any combination of these techniques.

The invention also includes a gaseous fuel burner as described above which also incorporates one or more pilot tubes which terminate in the fuel outlet space, the pilot tubes being so sized that, during use, they supply enough fuel to provide a pilot flame for re-ignition.

Preferably the pilot tube, or each pilot tube when there is more than one, terminates near the boundary between the fuel outlet space and the adjacent unpacked zone.

It is convenient for the production of a burner for the pilot tube, or each pilot tube when there is more than one, to terminate near the edge of burner. If it is desired to terminate a pilot tube in the centre of the burner it may pass through an unpacked zone provided that it terminates in the packed outlet zone.

In the case of liquid fuel burners (where high resistance to fuel flow is not important) the packing in the filel outlet space transfers the fuel by surface tension (i.e. it provides a wicking action). It is convenient to distinguish two mechanisms of fuel transfer. In the first mechanism the packing (which need not be porous) forms capillary sized channels, eg between different packing elements and/or between the packing and the combustion air tubes. In the second mechanism the packing is porous and transfer is achieved in the way a sponge soaks up water. Both mechanisms may operate at the same time. So far as fuel transfer is concerned all the packings suitable for use in gaseous fuel burners are also suitable for use in the fuel outlet space of liquid fuel burners.

However improved operation is achieved when the packing in the fuel outlet space assists heat transfer to the fuel thereby improving vaporisation of the fuel into the combustion zone. Thus it is preferable to use nonparticulate packings or particulate packings with good heat contact between the particles, e.g. those (mentioned above with reference to gas burners) in which the particles are bound together as by pressure sintering, heat sintering or the use of a binder.

(In optimising the performance of the burners described above the cross sectional area and density of packing of the combustion air tubes are particularly important. The cross sectional area of the combustion air tubes affects their resistance to air flow and reducing this area clearly increases the resistance. The density of packing defines the lengths of the diffusion paths and short paths assist the mixing of "the fuel and oxygen; the cross sectional area is also relevant in that it limits the number of tubes which can be packed into a given area. We have found that optimum dimensions are usually achieved when the bore of each combustion air tube is 0.0l-l.0 cm where it opens into the combustion zone and the bores of the tubes account for at least 25%, particularly at least 50%, of the surface of the fuel outlet space adjacent to the combustion zone.)

The invention includes combustion appliances which incorporate one or more burners as described above, e.g. a burner mounted in a combustion chamber adapted to be connected to a flue.

As a first example of such a combustion device we quote a device intended for heating a fluid, e.g. a central heating boiler, which also comprises a heat exchanger positioned so as to receive hot gases when the combustion appliance is alight.

As a second example we quote a radiant heat combustion device which also comprises a ceramic element positioned in its combustion zone so that, when the device is alight, combustion heats the element. This combination is particularly suitable for furnace construction, i.e. the radiant elements form sides and/or bottom and/or the top of the hot box. (It is clearly desirable to surround, in three dimensions, the hot space with radiant elements but it is necessary to provide an opening for flue gas and a door. If desired the door could take the form of movable bumer/element combinations.)

The invention will now be described, by way of example, with reference to the diagrammatic drawings accompanying this specification in which:

FIG. 1 is a perspective view, with part cut away, of a burner according to the preferred embodiment of the invention,

FIG. 2 is a horizontal cross-section through the burner shown in FIG. 1,

FIG. 3 is a vertical cross-section through the burner shown in FIG. 1,

FIG. 4 is top view of a burner having pilot tubes which terminate near the edge of the burner,

FIG. 5 is a vertical cross-section through the burner shown in FIG. 4,

FIG. 6 is a top view of a burner in which the pilot tubes terminate in the centre of the burner,

FIG. 7 is a vertical cross section through the burner shown in FIG. 6,

FIG. 8 is a vertical cross section through a liquid fuel burner according to the invention, and

FIG. 9 is a cross-sectional view of a burner as illustrated in FIGS. 1-8 installed in a water heater. (Note. FIGS. 1, 2, 3 and 9 were first filed with UK. patent application 46172/67 when they were identified as FIGS. 1, 2, 3, and 4. FIGS. 4, 5, 6 and 7 were first filed with U.I(. patent application 28732/68 when they were identified as FIGS. 1, 2, 3 and 4. FIG. 8 was first filed accompanying this specification.)

The burner shown in FIGS. 1-3 is a gas burner which comprises a fuel chamber 10 through which a plurality of combustion air tubes 11 pass. The fuel chamber is divided into a fuel inlet space 12 and a fuel outlet space 13 which is packed with a powder (whose particles were not bound together) to increase its flow resistance, said packing being supported by means of a partition 14. Each of the combustion air tubes 11 passes through a hole in the partition 14 and the size of the hole is such that an annular fuel duct 15 is formed around each of the combustion air tubes; these arrangements are most clearly seen in FIGS. 1 and 2. FIG. 3 also shows that the combustion air tubes 11 are secured infuel tight manner into circular holes in an air inlet zone plate 16.

In the use of the burner fuel enters the fuel inlet space 12 via the fuel supply line 17 and it passes through the interstices between combustion air tubes 11. From the fuel inlet space 12 the fuel passes through the packed fuel outlet space 13 into the combustion zone. Since the packing offers a relatively high resistance to the fuel gas flow and the interstitial space of the fuel inlet space 12 offers a relatively low resistance the construction encourages a uniform supply of fuel into the combustion zone.

A burner of the type just described was tested in the laboratory under an 8 inch chimney using, in separate tests, methane and town gas as the fuel. (Town gas has a variable composition but it always contains a substantial proportion of hydrogen, usually over 50% by volume.) The burner comprised 38 combustion air tubes 11 each having an outside diameter of 0.186 inches and an internal diameter of 0.175 inches, with triangular spacing 0.216 inches between centers. The overall dimensions of the burner were 1.8 by 1.0 inches by inch deep. It was fitted with a partition 14 /8 inches from the combustion zone. Each combustion air tube passed through a hole in-the partition and (except at the edge) the holes had a diameter of 0.197 inches; i.e. an annulus of 0.006 inches if uniform. The edge holes had a diameter of 0.189 inches, i.e. an annulus of 0.0015 inches if uniform. This restriction at the edges corrected a tendency to burn fuel rich at the edges of the combustion zone. The space between the partition and the combustion zone was packed with fused alumina ground to a powder having a particle size passing 16-25 mesh.

Although the burner was designed to burn gaseous fuels it was also able to burn kerosine (under a 17 inch chimney); the following maximum heat outputs (i.e. without flames coming away from the burner in the case of methane and town gas) were achieved:

The burner used in the experiment had its fuel outlet space 13 directly open to the combustion zone and the flow of fuel tended to be aligned with the flow of air. It was found possible to increase the maximum heat output given with methane by fitting a baffle 18 to deflect the fuel flow into the air flow. The baffle consisted of a perforated plate which covered the top of the fuel outlet space and left the combustion air tubes unobstructed. When the baffle was placed in contact with the fuel outlet space the maximum heat output with methane was increased from 2,000 (see the table above) to 2,500 BUT/sq.ins/hr. (Note the packing was a free flowing powder but, owing to the shape and size of its particles, it remained in the fuel outlet space 13 if the burner was turned on its side. It is possible that the heat of the flame results in partial heat-sintering thereby creating extra mechanical stability.)

The gas burner shown in FIGS. 4 and 5 is similar to that shown in FIGS. 13 but it also comprised a pilot tube 18 which bifurcates and terminates in outlets 18a and 18b which are positioned at the edge of the packed outlet zone just above the partition 14.

The pilot jets supply about 12% of the maximum rate of fuel (gas) utilisation. When the main fuel supply is off" the fuel from the pilot tube 18 percolates upwards through the fuel outlet space 13 and it burns in the combustion zone. Even though the flame is situated at one edge of the combustion zone it provides satisfactory re-ignition when the main fuel supply comes on.

In the burner shown in FIGS. 6 and 7 two pilot tubes 18a and 18b pass through the fuel inlet space 13 just below the partition 14 and turn upwards in the middle of the burner to terminate in the fuel outlet space 12 just above the partition 14.

The pilot flame burns in the same way as the one described with reference to FIGS. 4 and S'but it gives quicker re-ignition. This is an advantage in the case of burners with a large cross sectional area.

The pilot flames in burners as shown in FIGS. 57 do not burn in contact with fine pilot jets so that these jets cannot be blocked by sooting.

FIG. 8 illustrates a liquid fuel burner (similar to the burner illustrated in FIGS. 1-3) which contains a nonporous packing in the form of unglazed ceramic tubes 19 one of which is positioned around each of the combustion air tubes 11 and supported on projections 20. (FIG. 8 shows the arrangement of two adjacent tubes in the plane of their axes.)

The burner comprised 47 combustion air tubes 11 each having an outside diameter of 0. 1 86 inches, an inside diameter of 0.175 inches and a height of 1.25 inches. The tubes were arranged with triangular spacing with 0.245 inches between centers.

Each combustion air tube 11 was surrounded by an unglazed ceramic tube 19 with an internal diameter of 0.196 inches, an outside diameter of 0.240 inches and a height of 1 inch supported on projections 20 0.25 inches from the inlet end of the combustion air tube 11. This gives an annulus between the combustion air tube and the ceramic tube of 0.005 inches if uniform.

The burner was operated using kerosine as the fuel supplied by a constant level controller set to a level 16 mm below overflow. Capillary action raised fuel to the top of the combustion air tubes without danger of overflow. The heat output was 900 BTU/hour/square inch. (The same burner with the ceramic tubes removed gave a heat output of 500 BTU/hour/square inch with the fuel level 6 mm below overflow.)

FIG. 9 illustrates a water heater, suitable for use in a central heating appliance, which incorporates a burner 30 as shown in FIGS. 1-8. The water heater comprises the burner 30 installed at the base of the combustion space 31 connected to a flue at the outlet 32. During combustion the burner obtains its combustion air from the air space 33 and the hot gases produced by the combustion pass over one side of the heat exchanger 34 which heats the water circulated through the pipes 35 and 36. The burner obtains its fuel via a regulating device 37.

In the case of a gas burner the regulating device 37 is a constant pressure regulator and the heat output is controlled by interupting the gas supply as necessary. The burners shown in FIGS. 4-7 have pilot lights; otherwise a pilot jet 38 is provided.

In the case of a liquid fuel burner the regulating device 37 can be either a flow or level controller which adjusts the fuel supply in accordance with the heat requirement.

The burners described in the examples gave silent blue flame combustion by means of a diffusion mechanism. In all cases the flames extended less than about 5 mm from the burner.

We claim:

'1. A diffusion flame burner for fluid fuels, said burner comprising means fonning a fuel chamber having a fluid-tight bottom portion defining an air-inlet zone, a top portion defining a-combustion zone, and a fluidtight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from saidbottom portion to said top portion for conducting combustion air through said chamber from said air-inlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into a sequence of fuel spaces in superposed relation to each other between said bottom portion and. said top portion:

a. at least one fuel space of said sequence being an unpacked fuel space which provides a low resistance to the flow of fuel across the burner,

b. at least one other fuel space of said sequence being a packed fuel space in which the packing is of a type having capillary size channels and is adapted to control the flow of fuel towards the combustion zone, the first fuel space of said sequence beingunpacked and constituting a fuel inlet space adapted to be connected to a fuel supply and the last fuel space of said sequence being packed and constituting a fuel outlet space communicating with the combustion zone, the packing being exposed to the ambient and being unobstructed across its upper surface whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel andair mixing only in said combustion zone and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, and

c. the bore of each combustion air tube being 0.011.0 cm where it opens into the combustion zone and the bores of the tubes accounting for at least 25% of the surface of said fuel outlet space adjacent to the combustion zone.

2. Abumer according to claim 1, in which all the combustion air tubesare cylindrical.

3. A burner according to claim 2, in which the combustion air tubes are arranged with their axes parallel to one another.

4. A burner according to claim 1, which comprises a platewhich extends across the fuel chamber, permits the passage of fuel and which supports the packing.

5. A gaseous fuel burner according to claim 1, in which the packing in the packed fuel outlet space is a porous material which creates a high resistance to fuel flow through the zone.

6. A gaseous fuel burner according to claim 5, in which the porous mate rial is a particulate material.

7. A gaseous fuel burner according to claim 6, in which the particulate material is a powder whose particle size and particle density is such as to produce the required high resistance to fuel flow.

8. A gaseous fuel burner according to claim 7, in which the particles of the powder are bound together.

9. A burner according to claim 1 in which the bores of the tubes account for at least 50% of the surface of said fuel outlet space adjacent the combustion zone.

10. A liquid fuel burner according to claim 1, in which the packing has sufficient thermal conductivity to assist vaporisation.

11. A diffusion flame burner for fluid fuels, said burner comprising means forming a fuel chamber having a fluid-tight bottom portion defining an air inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting combustion air through said chamber from said air-inlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into two fuel spaces in superposed relation to each other between said bottom portion and said top portion:

a. the bottom fuel space being an unpacked fuel space which provides a low resistance to the flow of fuel across the burner,

b. the top fuel space being a packed fuel space in which the packing is of a type having capillary size channels and is adapted to control the flow of fuel towards the combustion zone, said unpacked space constituting a fuel inlet space adapted to be connected to a fuel supply and said packed fuel space constituting a fuel outlet space communicating with the combustion zone, the packing being exposed to the ambient and being unobstructed across its upper surface whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel and air mixing only in said combustion zone and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, and

c. the bore of each combustion air tube being 0.0l-1.0 cm where it opens into the combustion zone and the bores of the tubes accounting for at least of the surface of said fuel outlet space adjacent to the combustion zone.

12. A liquid fuel burner according to claim 11, in which the packing in the packed fuel outlet space is a porous material which conveys fuel to the combustion zone by means of capillary action.

13. A liquid fuel burner according to claim 12, in which the porous material is a particulate material whose particles are bound together.

14. A liquid fuel burner according to claim 12, in which the packing has sufficient thermal conductivity to assist vaporisation.

15. A burner according to claim 6 in which the bores of the tubes account for at least 50% of the surface of said fuel outlet space adjacent the combustion zone.

16. A diffusion flame burner for fluid fuels, said burner comprising means forming a fuel chamber having a fluid-tight bottom portion defining an air-inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting combustion air through said chamber from said air-inlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into a sequence of fuel spaces in superposed relation to each other between said bottom portion and said top portion:

a. at least one fuel space of said sequence being an unpacked fuel space-which provides a low resistance to the flow of fuel across the burner,

b. at least one other fuel space of said sequence being a packed fuel space in which the packing forms capillary sized channels which convey fuel to the combustion zone by means of capillary action and is adapted to control the flow of fuel towards the combustion zone and comprises unglazed ceramic tubes positioned around the combustion air tubes, the first fuel space of said sequence being unpacked and constituting a fuel inlet space adapted to be connected to a fuel supply and the last fuel space of said sequence being packed and constituting a fuel outlet space communicating with the combustion zone, whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel and air mixing only in said combustion zone and the resulting fuelair mixture burning as a diffusion flame in said combustion zone, and

. the bore of each combustion air tube being 0.0l-l.0 cm where it opens into the combustion zone and the bores of the tubes accounting for at least 25% of the surface of said fuel outlet space adjacent to the combustion zone.

17. A liquid fuel burner according to claim 21 in which the combustion air tubes are provided with projections on which said ceramic tubes are supported. 

1. A diffusion flame burner for fluid fuels, said burner comprising means forming a fuel chamber having a fluid-tight bottom portion defining an air-inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting combustion air through said chamber from said airinlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into a sequence of fuel spaces in superposed relation to each other between said bottom portion and said top portion: a. at least one fuel space of said sequence being an unpacked fuel space which provides a low resistance to the flow of fuel across the burner, b. at least one other fuel space of said sequence being a packed fuel space in which the packing is of a type having capillary size channels and is adapted to control the flow of fuel towards the combustion zone, the first fuel space of said sequence being unpacked and constituting a fuel inlet space adapted to be connected to a fuel supply and the last fuel space of said sequence being packed and constituting a fuel outlet space communicating with the combustion zone, the packing being exposed to the ambient and being unobstructed across its upper surface whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel and air mixing only in said combustion zone and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, and c. the bore of each combustion air tube being 0.01-1.0 cm2 where it opens into the combustion zone and the bores of the tubes accounting for at least 25% of the surface of said fuel outlet space adjacent to the combustion zone.
 2. A burner according to claim 1, in which all the combustion air tubes are cylindrical.
 3. A burner according to claim 2, in which the combustion air tubes are arranged with their axes parallel to one another.
 4. A burner according to claim 1, which comprises a plate which extends across the fuel chamber, permits the passage of fuel and which supports the packing.
 5. A gaseous fuel burner according to claim 1, in which the packing in the packed fuel outlet space is a porous material which creates a high resistance to fuel flow through the zone.
 6. A gaseous fuel burner according to claim 5, in which the porous material is a particulate material.
 7. A gaseous fuel burner according to claim 6, in which the particulate material is a powder whose particle size and particle density is such as to produce the required high resistance to fuel flow.
 8. A gaseous fuel burner according to claim 7, in which the particles of the powder are bound together.
 9. A burner according to claim 1 in which the bores of the tubes account for at least 50% of the surface of said fuel outlet space adjacent the combustion zone.
 10. A liquid fuel burner according to claim 1, in which the packing has sufficient thermal conductivity to assist vaporisation.
 11. A diffusion flame burner for fluid fuels, said burner comprising means forming a fuel chamber having a fluid-tight bottom portion defining an air inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting combustion air through said chamber from said air-inlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into two fuel spaces in superposed relation to each other between said bottom portion and said top portion: a. the bottom fuel space being an unpacked fuel space which provides a low resistance to the flow of fuel across the burner, b. the top fuel space being a packed fuel space in which the packing is of a type having capillary size channels and is adapted to control the flow of fuel towards the combustion zone, said unpacked space constituting a fuel inlet space adapted to be connected to a fuel supply and said packed fuel space constituting a fuel outlet space communicating with the combustion zone, the packing being exposed to the ambient and being unobstructed across its upper surface whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel and air mixing only in said combustion zone and the resulting fuel-air mixture burning as a diffusion flame in said combustion zone, and c. the bore of each combustion air tube being 0.01-1.0 cm2 where it opens into the combustion zone and the bores of the tubes accounting for at least 25% of the surface of said fuel outlet space adjacent to the combustion zone.
 12. A liquid fuel burner according to claim 11, in which the packing in the packed fuel outlet space is a porous material which conveys fuel to the combustion zone by means of capillary action.
 13. A liquid fuel burner according to claim 12, in which the porous material is a particulate material whose particles are bound togeTher.
 14. A liquid fuel burner according to claim 12, in which the packing has sufficient thermal conductivity to assist vaporisation.
 15. A burner according to claim 6 in which the bores of the tubes account for at least 50% of the surface of said fuel outlet space adjacent the combustion zone.
 16. A diffusion flame burner for fluid fuels, said burner comprising means forming a fuel chamber having a fluid-tight bottom portion defining an air-inlet zone, a top portion defining a combustion zone, and a fluid-tight side portion between said bottom portion and said top portion, said burner also comprising a plurality of combustion air tubes passing through said fuel chamber from said bottom portion to said top portion for conducting combustion air through said chamber from said air-inlet zone to said combustion zone in confined streams out of contact with fuel in said chamber, said fuel chamber being open to the atmosphere at its said top portion only and being divided into a sequence of fuel spaces in superposed relation to each other between said bottom portion and said top portion: a. at least one fuel space of said sequence being an unpacked fuel space which provides a low resistance to the flow of fuel across the burner, b. at least one other fuel space of said sequence being a packed fuel space in which the packing forms capillary sized channels which convey fuel to the combustion zone by means of capillary action and is adapted to control the flow of fuel towards the combustion zone and comprises unglazed ceramic tubes positioned around the combustion air tubes, the first fuel space of said sequence being unpacked and constituting a fuel inlet space adapted to be connected to a fuel supply and the last fuel space of said sequence being packed and constituting a fuel outlet space communicating with the combustion zone, whereby, during the use of the burner, air flows through the combustion air tubes into the combustion zone where it reacts with the fuel which flows upwardly and through said fuel outlet space into the combustion zone thereabove, said fuel and air mixing only in said combustion zone and the resulting fuelair mixture burning as a diffusion flame in said combustion zone, and c. the bore of each combustion air tube being 0.01-1.0 cm2 where it opens into the combustion zone and the bores of the tubes accounting for at least 25% of the surface of said fuel outlet space adjacent to the combustion zone.
 17. A liquid fuel burner according to claim 21 in which the combustion air tubes are provided with projections on which said ceramic tubes are supported. 